“Helicopter seeds,” officially known as samaras, are unique fruits with a flattened, wing-like structure. This design allows them to spin as they fall, mimicking a helicopter rotor. This enables wind to carry the seeds further from their parent tree, increasing their chances of survival and propagation. This spinning motion earned them their common nickname.
Common Trees with Helicopter Seeds
Many familiar trees produce these distinctive winged seeds, with maples (genus Acer) being perhaps the most well-known. Maple samaras typically appear in joined pairs, resembling a handlebar mustache. The size and angle of these paired wings vary among maple species, such as the one-inch red maple samaras or the larger two-inch silver maple samaras.
Beyond maples, other trees also produce samaras. Ash trees (genus Fraxinus) produce single-winged samaras, often called “keys,” which are elongated and hang in clusters. Elm trees (genus Ulmus) feature samaras where the seed is centrally located within a papery, often nearly round, wing. These can be green and coin-sized, appearing before the tree fully leafs out. The tree of heaven (Ailanthus altissima) also produces single samaras, which are 1 to 2 inches long and have a twisted tip, hanging in clusters. Common hoptree (Ptelea trifoliata) is another example, producing samaras that persist from late summer through winter.
The Mechanics of Samara Flight
The spinning descent of a samara is a sophisticated aerodynamic process called autorotation. As a samara detaches from the tree, its asymmetrical shape, with the heavier seed at one end and the wing extending from it, causes it to begin rotating. This rotation generates lift, slowing its vertical speed.
Research indicates that the spinning samara creates a stable leading-edge vortex (LEV) over its upper surface. This tornado-like vortex reduces air pressure above the wing, pulling the seed upward and providing a boost against gravity. This mechanism is remarkably similar to the aerodynamic principles used by hovering insects and hummingbirds. This allows the seed to remain airborne for longer, increasing its susceptibility to wind currents and enabling greater dispersal distances.
The Advantage of Wind Dispersal
Wind dispersal, or anemochory, offers evolutionary advantages for trees. One primary benefit is the ability to spread seeds far from the parent plant. This distance helps to reduce competition for light, water, and nutrients between the offspring and the parent tree, as well as among sibling seedlings. Without effective dispersal, seeds would often fall directly beneath the parent, leading to overcrowding and limited resources.
Dispersal also helps seeds escape density-dependent predators and pathogens, which often concentrate around mature parent trees. By traveling to new locations, seeds have a better chance of finding suitable growing conditions and colonizing new habitats. This broad distribution enhances its ability to adapt to changing environmental conditions.